1. Field of the Invention
This invention relates generally to steering controls for tracked vehicles such as tanks and more specifically to an automatically variable torque control for a hydrostatic drive unit which engages a neutral shaft in a steering mechanism.
2. Brief Description of the Prior Art
Prior steering mechanisms for tracked vehicles which include hydrostatic and/or hydrodynamic units for driving a neutral shaft have been disclosed in U.S. Pat. No. 3,371,734, issued Mar. 15, 1968 and in the following published German patent applications: DE-PS No. 860,601; DE-PS No. 1,480,725; DAS No. 1,929,380; DOS No. 23 22 457; and DAS No. 1,174,182. In these prior superimposed steering mechanisms, control of the hydrostatic drive unit through control of a hydrostatic pump regulating lever and/or an oil motor absorption quantity valve was provided as a function of the steering wheel position. As such, torque requirements of the steering drive mechanism and the torque characteristics of a hydrodynamic drive unit which may also engage the neutral shaft were not considered.
Since the torque requirements for the steering drive were not solely dependent upon the position of the vehicle steering wheel but were also a function of the friction between the chain tread and the terrain or road, it was necessary to provide such prior steering mechanisms with hydrostatic drive units designed for maximum output torque at minimum oil motor absorption quantity or for maximum oil motor output speed at full absorption quantity. In either event, the hydrostatic units required pumps and motors of large size and substantial weight and cost.